Increasingly stringent environmental regulations will require significant reductions in the sulfur content of transportation fuels. For example, by the end of this decade, maximum sulfur levels for distillate fuel will be limited to 10 wppm in Europe and Japan and 15 wppm in North America. To meet these ultra-low sulfur requirements without expensive modifications to existing refineries, it will be necessary to design a new generation of catalyst that has very high activity for desulfurization, particularly for distillate fuels at low to medium pressure.
In one approach, a family of compounds, related to hydrotalcites, e.g., ammonium nickel molybdates, has been prepared. Whereas X-ray diffraction analysis has shown that hydrotalcites are composed of layered phases with positively charged sheets and exchangeable anions located in the galleries between the sheets, the related ammonium nickel molybdate phase has molybdate anions in interlayer galleries bonded to nickel oxyhydroxide sheets. See, for example, Levin, D., Soled, S. L., and Ying, J. Y., Crystal Structure of an Ammonium Nickel Molybdate prepared by Chemical Precipitation, Inorganic Chemistry, Vol. 35, No. 14, p. 4191-4197 (1996). The preparation of such materials also has been reported by Teichner and Astier, Appl. Catal. 72, 321-29 (1991); Ann. Chim. Fr. 12, 337-43 (1987), and C. R. Acad. Sci. 304 (II), #11, 563-6 (1987) and Mazzocchia, Solid State Ionics, 63-65 (1993) 731-35.
Another approach is disclosed in U.S. Pat. Nos. 6,162,350; 6,652,738, 6,635,599 and 6,534,437, which relates to a family of bulk Group VIII/Group VIB trimetallic catalysts for the removal of sulfur from distillate fuels. The preferred trimetallic catalysts are comprised of Ni—Mo—W and are prepared from a variety of catalyst precursor compounds.
While some of the above mentioned catalysts have met with varying degrees of success, there is still a need in the art for ever more active catalysts to produce transportation fuels having ultra-low levels of sulfur, particularly for low pressure hydrotreating, e.g. a hydrogen partial pressure of less than 500 psig or less than 1000 psig.